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finish with endianness section

This commit is contained in:
Joe Ardent 2023-06-28 12:52:54 -07:00
parent e7bd1bd5f8
commit 64d5597802
1 changed files with 17 additions and 13 deletions
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content/rnd/a_serialized_mystery/index.md
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@ -188,7 +188,7 @@ impl Decode<'_, Sqlite> for DbId {
fn decode(value: SqliteValueRef<'_>) -> Result<Self, sqlx::error::BoxDynError> { fn decode(value: SqliteValueRef<'_>) -> Result<Self, sqlx::error::BoxDynError> {
let bytes = <&[u8] as Decode<Sqlite>>::decode(value)?; let bytes = <&[u8] as Decode<Sqlite>>::decode(value)?;
let bytes: [u8; 16] = bytes.try_into().unwrap_or_default(); let bytes: [u8; 16] = bytes.try_into().unwrap_or_default();
Ok(u128::from_be_bytes(bytes).into()) Ok(u128::from_ne_bytes(bytes).into())
} }
} }
``` ```
@ -313,25 +313,29 @@ not storing them in string from. Fundamentally, the ULID was a simple [128-bit p
integer](https://doc.rust-lang.org/std/primitive.u128.html), capable of holding values between 0 and integer](https://doc.rust-lang.org/std/primitive.u128.html), capable of holding values between 0 and
340,282,366,920,938,463,463,374,607,431,768,211,455. 340,282,366,920,938,463,463,374,607,431,768,211,455.
But there's a problem: we're storing our ID in the database as a sequence of 16 bytes. I was asking But there's a problem: I was storing the ID in the database as a sequence of 16 bytes. I was asking
for those bytes in "native endian", which in my case, meant "little endian". If you're not familiar for those bytes in "native endian", which in my case, meant "little endian". If you're not familiar
with endianness, there are two varieties: big, and little. "Big" makes the most sense; if you see a with endianness, there are two varieties: big, and little. "Big" makes the most sense for a lot of
number like "512", it's big-endian; the end is the part that's left-most, and "big" means that it is people; if you see a number like "512", it's big-endian; the end is the part that's left-most, and
the most-significant-digit. This is the same as what westerners think of as "normal" numbers. In the "big" means that it is the most-significant-digit. This is the same as what westerners think of as
number "512", the "most significant digit" is `5`, which correspends to `500`, which is added to the "normal" numbers. In the number "512", the "most significant digit" is `5`, which correspends to
next-most-significant digit, `1`, corresponding to `10`, which is added to the next-most-significant `500`, which is added to the next-most-significant digit, `1`, corresponding to `10`, which is added
digit, which is also the least-most-significant-digit, which is `2`, which is just `2`, giving us to the next-most-significant digit, which is also the least-most-significant-digit, which is `2`,
the full number `512`. which is just `2`, giving us the full number `512`.
If we put the least-significant-digit first, we'd write the number `512` as "215"; the order when If we put the least-significant-digit first, we'd write the number `512` as "215"; the order when
written out would be reversed. This means that the lexicographic sort of `512, 521` would have "125" written out would be reversed. This means that the lexicographic sort of `512, 521` would have "125"
come before "215", which is backwards. come before "215", which is backwards.
Little-endiannes is like that. If a multibyte value is on a little-endian system, the least-significant Little-endiannes is like that. If a multibyte numeric value is on a little-endian system, the
bytes will come first, and the sorting would be non-numeric. least-significant bytes will come first, and a lexicographic sorting of those bytes would be
non-numeric.
Unfortunaly, my computer is based on the Intel x86 instruction set, which means that it represents The solution, though, is simple: just write them out in big-endian order! This was literally a
numbers in "little endian" form. This means that one-line change in the code, to switch from `to_ne_bytes()` ("ne" for "native endian") to
`to_be_bytes()`.
Boom. Sorted.
## The actual problem ## The actual problem